278  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.        [bull.  260. 
of  Montana.  On  the  east,  beds  of  the  same  character  as  those  occur- 
ring in  the  Coeur  d'Alene  district  extend  to  the  Missoula  River  at  the 
mouth  of  the  St.  Regis  de  Borgia.  Here  there  is  apparently  some 
change  in  lithological  character,  but  quartzites  and  red  and  green 
siliceous  argillites,  probably  of  Algonkian  age,  extend  at  least  to  the 
town  of  Missoula,  in  Montana,  and  probably  for  some  distance  farther 
cast.  The  area  of  Algonkian  sediments  has  a  width  of  about  80 
miles  between  Coeur  d'Alene  Lake  and  the  Missoula  River,  and  it  is 
probable  that  extensive  exposures  of  Algonkian  beds  continue  100  or 
more  miles  to  the  eastward,  connecting  the  Coeur  d'Alene  area  with 
the  known  Algonkian  areas  of  central  and  northern  Montana. 
On  the  south,  Lindgren a  has  shown  that  the  sedimentary  rocks 
near  Lolo  Pass,  which  are  probably  part  of  the  same  series  that  pre- 
vails in  the  Coeur  d'Alene  Mountains,  are  cut  off  by  the  great  gra- 
nitic batholith  of  central  Idaho. 
In  the  Coeur  d'Alene  district  alone,  the  Algonkian  rocks  have  a 
thickness  of  at  least  10,000  feet,  and  it  is  certain  that  by  no  means 
the  entire  series  of  rocks  of  this  age  developed  in  northwestern  Mon- 
tana is  represented. 
As  a  whole,  the  Algonkian  sediments  of  the  Coeur  d'Alejie  region 
exhibit  little  lithological  contrast.  They  are  chiefly  shallow-water 
deposits,  as  shown  by  the  prevalence  of  ripple  marks  and  sun  cracks. 
They  comprise  dark  argillites  (mud  rocks),  graywackes  (mud-sand 
rocks),  quartzites  of  various  degrees  of  coarseness,  and  usually 
sericitic,  quartzitic  sandstones,  and  impure  limestones  or  calcareous 
argillites.  Secondary  cleavage  is  frequently  present  in  all  but  the 
coarser  arenaceous  beds,  but  this  slaty  structure  varies  greatly  in 
development  in  different  parts  of  the  field. 
No  sediments  younger  than  the  Algonkian  occur  in  the  Coeur 
d'Alene  district,  with  the  exception  of  fluviatile  deposits,  some  of 
which  may  be  of  Tertiary  age. 
The  Algonkian  rocks  are  cut  by  a  number  of  masses  of  syenite 
most  of  which  have  the  form  of  small  intrusive  stocks,  and* by  a  few, 
dikes  of  diabasic,  dioritic,  and  minette-like  habit,  which  have  not 
yet  been  studied.  The  principal  syenitic  intrusions  occur  northeast 
of  Wallace  along  a  northeast-southwest  line  (see  fig.  17).  There- 
is  no  means  of  determining  the  age  of  these  intrusions.  If  the 
syenitic  stocks  were  intruded  at  the  same  time  as  the  great  granitic 
batholith  of  central  Idaho,  they  are  of  post-Triassic  and  pre-Miocenej 
age. 
Divisions  and  lithology  of  the  sedimentary  rocks. — Owing  to  the 
rather  monotonous  sequence  of  the  Algonkian  sediments,  and  the 
absence  of  fossils  or  unconformities,  division  of  the  group  into  dis- 
°  A  geological  reconnaissance  across  the  Bitterroot  Range  and  Clearwater  Mountains 
in  Montana  and  Idaho:    Prof.  Paper  U.  S.  Geol.  Survey  No.  27,  1904,  p.  1G. 
